LEDs are increasingly used for various illumination applications. Many of such application require an LED driver to be dimmable to vary the amount of light produced by the LED or LEDs. Further, in a combination of LEDs producing light of different colors, where the different colors are mixed to obtain light with a specific color temperature, an LED driver for LEDs of one color or several colors also needs to be dimmable.
Existing converters used for dimmable LED driver purposes are mainly hard switching converters. Use of hard switching converters has disadvantages in that they do not have a high efficiency, and in that they generate a relative high level of electromagnetic interference, EMI.
A relatively low efficiency of the hard switching converter leads to increased power dissipation at the commonly used high converter frequencies. As a result, cooling of some converter power components becomes a major problem, which restricts a desired miniaturization of the converter.
Generating a relative high level of EMI means that relative large area has to be reserved for mains filtering, which further restricts miniaturization of the converter.
LED drivers are commonly dimmed using pulse width modulation, PWM, which e.g. is disclosed in reference U.S. Pat. No. 6,510,995 using a resonant converter. For very small duty cycles in PWM operation, PWM controlled converters cannot provide a desired stability of the duty cycle. An unstability of the duty cycle may result in flickering, which is undesirable.